198 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



There are no indications of eyes on the cephalon. A narrow 

 median longitudinal ridge occurs on the posterior half of each of the 

 seven segments of the thorax and on an eighth segment that appears 

 to be still attached to the pygidium. 



This little species, i6 mm. in length, appears to be generically 

 identical with Mollisonia symmetrica, which occurs at about the same 

 geological horizon a few miles distant. It is much smaller and more 

 slender than that species. 



Formation and locality. — Middle Cambrian: (35k) Burgess shale 

 member of the Stephen formation (phyllopod bed), on the west slope 

 of the ridge between Mount Field and Wapta Peak, one mile (1.6 

 km.) northeast of Burgess Pass, above Field, British Columbia. 



MOLLISONIA ? RARA, new species 



Plate 24, figs. 6 and 7 



Of this species there are several fragmentary specimens. The 

 species differs from M. gracilis, with which it is associated, in the 

 character of the thoracic segments and pygidium ; also, so far as we 

 can determine from this superficial study, there are seven segments 

 and the pygidium shows distinct segmentation with a denticulated 

 border. ' ' '^l 



The specimen illustrated by fig. 6 indicates that the species was 

 the largest of the genus and may have had a length of from 5 to 6 cm. 



Formation and locality. — Middle Cambrian : (35k) Burgess shale 

 member of the Stephen formation (phyllopod bed), on the west slope 

 of the ridge between Mount Field and Wapta Peak, one mile (1.6 

 km.) northeast of Burgess Pass, above Field, British Columbia. 



TONTOIA, new genus 



The generic description is included with that of the type species. 



Genotype. — Tontoia kwaguntensis, new species. 



Stratigraphic range. — The stratigraphic range is limited to the 

 upper portion of the Tonto sandstone on the surface of a few thin 

 layers of sandstone. 



Geographic distribution. — The species has been found only in 

 Kwagunt Valley off the eastern end of Kiabab Plateau in the Grand 

 Canyon of the Colorado, Northern Arizona. 



The generic name is derived from Tonto, an Indian name applied 

 to the basal sandstone of the Cambrian series in the Grand Canyon 

 region. 



